1. Field of the Invention
The present invention is broadly concerned with a method for optically measuring a selected characteristic of a patient's blood or tissue by initially passing a current through a relatively short tissue segment of a patient in order to significantly increase blood flow therethrough, followed by transcutaneous illumination so as to develop information about a blood or tissue characteristic. Preferably, the invention is used in the context of pulse oximetry in order to generate a higher amplitude signal, thus giving more accurate results.
2. Description of the Prior Art
A common method of measuring blood gases (e.g., blood oxygen) involves taking a blood sample and examining it in a clinical blood gas analyzer. There are numerous drawbacks to this type of procedure. First, it is invasive and poses a risk of infection to the patient. Second, this method requires trained clinical personnel to sample and handle the potentially infectious blood. Finally, this approach provides only intermittent information about the blood components rather than continuous, up-to-the-minute measurements.
Non-invasive methods have been employed in order to avoid some of the above problems. One such method involves transcutaneous monitoring of physiological characteristics and components of the blood or tissue. In this technique, a portion of a patient's arterial blood-perfused tissue is transcutaneously illuminated and detected using light of two or more wavelengths. For example, changes in the amount of arterial blood in the tissue during a blood pressure pulse alters the amount and character of the light detected by the sensor's photodetector. The amounts of light transmitted through the tissue of each wavelength are then compared to calculate desired blood gas characteristics. In the case of pulse oximetry, this measurement determines to what degree the arterial blood flowing through the tissue is saturated with oxygen.
The quality of such measurements depends in part on the concentration of arterial blood in the portion of tissue illuminated by the sensor, and in part on the magnitude of the pulsatile changes in the amount of blood in the tissue. Accordingly, it is desirable to select advantageous sensor sites on the body known to have a high concentration of blood vessels (e.g., the fingers), or take other measures to insure a high blood flow concentration at the site. For example, prior practices have involved use of a chemical irritant applied topically at the sensor site to increase blood flow, with or without the application of heat. However, these expedients can cause excessive skin irritation when used for extended periods of time and thus are not practical for extended, continuous measurements. Furthermore, these effectively limit the choice of sensor sites, usually to the earlobes and fingertips.
Another prior approach for increasing arterial blood flow is embodied in the so-called "Cyclops" device used in the early 1960s. The Cyclops detects blood by measuring the amount of light reflected from the skin and is preferably placed in the middle of the forehead. The skin is first given a treatment using histamine phosphate, a counter irritant which has the effect of producing vascular dilation (when blood vessels are dilated, more blood will flow thus providing a larger signal to the sensor). A voltage is then applied to the skin to drive the histamine phosphate into the tissue by a process of iontophoresis. Subsequently, a compression plate is applied to the skin to render it bloodless and enable measurement of a baseline light reflection value. Thereafter, blood is allowed to flow to the dilated vessels, and measurements are taken of the blood oxygen level.
Despite all of these prior efforts, there remains a need in the art for an improved process which will enhance blood flow through tissue so as to increase the amplitude and strength of characterization signals, and especially pulse oximetry signals.